Give Vibratory Screens A Fair Shake

Understand the key factors in their selection and use

By Amin Almasi, mechanical consultant

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Plants rely on vibrating screens for a variety of duties such as separating, sifting, sorting, dewatering and classifying. Some vibrating screens, if designed properly — i.e., with a full range of adjustments, even can serve as feeders. Today’s devices generally boast higher efficiency, lower energy consumption and better reliability than traditional ones, thanks to improved design techniques and accumulated knowledge and know-how. However, success in an application still depends upon proper selection of screens, vibrating mechanisms and auxiliaries.

Many chemical processes require control of the size characteristics of a feed material. The vibrating screen is the most widely used device to split a feed of particulate materials into different grades of coarse and fine materials. Depending upon the degree of material separation and classification required by an application, designs may include two or three decks. In many units, oversize material goes to size reduction equipment for processing to an acceptable size. Even the most efficient vibrating screen will retain some undersize material.

Design, Sizing And Selection

A vibrating screen consists of many different components, e.g., a frame, vibrating mechanism, springs, screen deck(s), liners, etc. Six factors — width, length, screen inclination angle, vibration frequency, vibration amplitude and vibration pattern — are important in the design and operation of vibrating screens. Two parameters — capacity and efficiency — usually define the performance of any vibrating screen. These performance parameters aren’t independent; efficiency usually varies inversely with capacity. The screen capacity closely relates to the width. The screen length mainly affects the screening efficiency; the efficiency generally increases with length. The efficiency also depends, to a lesser degree, on the inclination angle, with efficiency declining as the angle gets steeper.

Efficiency could reach up to ≈95% using a screen with large dimensions and the best technologies for everything. This rarely has been attained and requires dedicated and costly tests. In practice, efficiencies of 80–90% are more realistic when using optimized designs. Many modern, properly designed vibrating screens achieve efficiencies of ≈85–88%.

Length-to-width ratio typically is 2.5:1 to 3.5:1.It’s not common to achieve efficiencies greater than 80% using a relatively short screen (say, with a ratio of ≤2:1) or angles >30°. Many ordinary and low-cost vibrating screens only attain efficiencies of ≈65–80%, even though their manufacturers claim high efficiencies. Therefore, it’s essential to properly evaluate vibrating screen designs and verify claimed performance and parameters.

Inclined screens commonly are used. Many modern vibrating screens rely on an angle between 10° and 25°. In many inclined screens, a single unbalance, rotating on a horizontal axis, generates a simple motion (circular or elliptical) mainly in the vertical plane. This motion imparts little positive movement to particles. Movement mainly stems from the screen inclination and the force of gravity, which cause the particle mass to travel at velocities of ≈0.3–0.6 m/s.

Angles lower than 10° aren’t common in conventional vibrating screens. Some specially designed linear-stroke vibrations can allow placing the vibrating screen at shallow angles (say, <10°). However, conventional screens with a circular or elliptical type of vibratory motion often require >15° angles. Screens set at a shallow angle or near horizontal usually employ a pair of unbalances, rotating in opposite directions on parallel horizontal axes, to generate a (nearly) straight-line reciprocating motion, inclined to the plane of the screen surface at 40°–50°. Velocities on a horizontal surface typically run ≈0.3–0.5 m/s but can be increased if necessary by inclining the screen downward (say, ≈10–15°). On the other hand, steep angles (>35°) aren’t common. Always check that a design allows convenient slope adjustment.

Vibration screens come in standard models as well as custom-engineered versions. In standard models, the frame and other major components are fixed; only the screen and a few other items are tailored for each application. This enables relatively fast delivery. In contrast, custom-engineered units are expensive and incur long lead times, making them sensible only for special situations.

Although vibrating screens have been widely used for many decades, engineers and operators at chemical plants often possess only limited knowledge of their design, installation and operation. Generally, the screening operation is complex. Theoretical models offer little practical utility. So, instead, for design, selection and sizing, users typically rely on empirical curves and formulae provided by manufacturers in conjunction with information on the feed rate, particle sizes, etc., to determine the type of screen deck and details such as the width and length of the screen, screen material, aperture size and percentage open area. Each major manufacturer has developed its own set of curves and formulae. However, many textbooks and handbooks present the curves and formulae from the same well-known manufacturer.

Operational flexibility is important to deal with changes in feed materials or processing conditions as well as emergency situations. For instance, proper operational parameters can enable screening of different feed materials such as sticky and adhesive bulk solids by utilizing automatic and repetitive pulsing vibrations.

Besides classification, another well-known application of vibrating screens is for dewatering. This can involve two different tasks: separating free water and removing surface moisture from the wet materials. The rate of dewatering usually depends on the instantaneous moisture content of wet materials as well as the amplitude of the vibration. In general, a vibrating screen with larger vibration amplitudes provides better dewatering performance.

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